1. Field of the Invention
The present invention relates to an improvement of a developing device for a color image forming apparatus, for example, a copier, a facsimile machine, a printer, etc.
In particular, the present invention relates to a revolving type developing device having a plurality of developing units, which is composed of a developing roller and a mono-color toner (i.e., cyan, magenta, yellow and black).
2. Discussion of the Related Art
A conventional developing apparatus for a color image forming apparatus has a revolving type developing device, in which a plurality of developing units each having a different mono-color toner are installed and disposed adjacent to a photo-conductive drum to develop a latent image formed thereon.
In such a device, after one of the mono-color developing processes is completed, the developing device revolves about itself and faces another developing unit to the photoconductive member for development with different mono-color toner. Thereby, an excessive scale of the developing device is avoided due to the form of the developing device, as shown in the specification of Japanese Published Utility Model Application Number 52-110442/77 and Japanese Published Patent Application Number 58-172660/83.
However, in the developing device disclosed in the former specification, a driving gear disposed in the copier body and a thrusting gear disposed in the developing unit interfere with each other when both gears are engaged with each other. Therefore, the mechanism has a drawback in that the driving force cannot be transmitted precisely from the driving source to the developing device.
Furthermore, if both gears can engage smoothly and precisely, the connection between them cannot be released freely at the position where the engagement is made due to the existence of a cam plate rigidly fixed to the body.
In addition, in the developing device disclosed in the latter specification, the developing gear and driving gear conflict with each other when the engagement between them is made. Furthermore, if it is kept going even in such a condition, namely the developing gear is rotated regardless of such an engagement condition, the teeth of the gears are broken and, as a result, the engagement of them is broken.
Also, if a transmitting device of the driving force (for example, the driving gear mentioned above) is fixed rigidly at the body of the image forming device and there exists an unevenness of the size in the transmitting tools (for example, the driving gear disposed in the copier body and the thrusting gear disposed in the developing unit), the rotational speed of the transmitting gear becomes uneven and the gear begins vibrating. This adversely effects the engaging condition. As a result, unevenness of the toner image, unevenness of the toner density and scattering of the toner in the copy sheet are introduced under the above condition of developing. Also, an unevenness of the rotation speed of the developing roller and the vibration of the developing device are caused by an unevenness of the rotation of the gear.
Additionally, in the revolving type developing device as described in the above-mentioned specification, it is difficult to make a positional accuracy when one of the developing units is positioned against the photoconductive member by the action of revolving and positioning thereof. Therefore, a cost of the parts for the positioning means and the driving force transmitting devices rises and the structure thereof becomes complex.